Polysilazanes are generally known and are used in particular as intermediates for silicon carbonitride and silicon nitride moldings. For example, EP-A-235 486 describes a process for preparing polysilazanes in which the product obtained from the reaction of R.sub.3 SiCl, R.sub.2 SiCl.sub.2 or R.sub.4 Si.sub.2 Cl.sub.2 with an amine is reacted with organotrichlorosilanes and then again with an amine. A disadvantage of this process is that the polysilazanes have relatively low thermal stability, i.e., the polymers are highly crosslinked by subsequent crosslinking and may become insoluble and infusible during preparation or during storage. The resultant polysilazanes are unsuitable for a shaping process, such as, for example, fiber spinning.
Therefore, it is an object of the present invention to provide a process for preparing polysilazanes which have good thermal stability. A further object of the present invention is to provide a process for preparing polysilazanes which are suitable for fiber spinning.